A spark across a lab bench proved electromagnetic waves travel through empty air

Between 1886 and 1889, Heinrich Hertz built a simple spark-gap transmitter and a loop of wire as a receiver, and showed that sparking one produced a matching spark in the other across open air, proof that electricity could radiate as a wave exactly as Maxwell's equations had predicted two decades earlier. Hertz measured the waves' speed and confirmed it matched light's. He never patented the discovery; the unit of frequency, the hertz, now carries his name.

— Heinrich Hertz, Untersuchungen über die Ausbreitung der elektrischen Kraft — 1888